The present invention relates to a variable-focus lens and more particularly to a lens involving the deformation of a body of liquid (“drop”) by electrowetting effects.
A variable-focus lens usually comprises an enclosure, bounded by two transparent windows, which contains at least two immiscible liquids of different refractive indices. The two liquids are in contact over a moveable refractive interface through which the light rays received by the lens pass. The liquid lens includes a system for deforming the moveable refractive interface by electrowetting effects, thus making it possible to modify the optical power of the lens.
The housing for such a lens generally constitutes a rigid structure. The pressure of the liquids in the housing may increase substantially, for example, during the operations of assembling the components of the housing, or, once the housing has been assembled, upon an increase in temperature of the liquids of the lens, which have higher expansion coefficients than the expansion coefficients of the constituent materials of the housing.
Excessive pressure of the liquids contained in the housing increases the risk of causing the transparent plates to deform, resulting in an undesirable optical distortion. In the worst case, if the increase in pressure of the liquids is too high, this may result in fracture of the transparent plates. Special precautions therefore have to be taken when assembling the mount for the lens and/or to limit the temperature range permitted for storing and using such a lens.
Patent application U.S. Ser. No. 11/284125, which is commonly owned and not yet published (not prior art), describes a housing for a variable-focus lens that includes a compensating device for the expansion of the liquids contained therein. The disclosure of this prior application is incorporated by reference into the present application.
FIG. 1 is substantially similar to FIG. 3 of patent application U.S. Ser. No. 11/284125 and shows a variable-focus lens mount 10, having an optical axis Δ, which comprises an upper part 12 and a lower part 14 which, when they are assembled, define an internal volume 15. The lower part 14 comprises a body 16 having a base 17 through which a central opening 18 passes, the base being extended by a cylindrical lateral portion 20. The base 17 comprises a corrugated portion 23, the cross section of which in a plane containing the axis Δ has the exact or approximate form of an “S”. A transparent cylindrical plate 24 is fastened to the body 16 by adhesive 22. The upper part 12 of the mount 10 comprises a cover 30 through the central part of which a cylindrical opening 32 passes. The upper part is extended by a cylindrical lateral wall 34. The cover 30 includes an elastic portion 36 provided between the opening 32 and the cylindrical lateral wall 34. The elastic portion 36 comprises a corrugated portion, the cross section of which in a plane containing the axis Δ has the exact or approximate form of an “S”. A transparent cylindrical plate 38 is fastened to the cover 30 by adhesive 40. An intermediate piece 42 is placed in the internal volume 15 in electrical contact with the body 16. Passing through the intermediate piece 42 is an opening that defines a truncated conical surface 48 adjacent to the glass plate 24. The intermediate piece 42 is made of a conducting material and is covered with an insulating layer 49 on the surfaces in contact with the liquids. A seal 50 is placed between the body 16 and the cover 30.
A volume (“drop”) of an insulating liquid 52 is placed on the conical surface 48, and the rest of the internal volume 15 is filled with an electrically conducting liquid 54, which is immiscible with the insulating liquid, has a different refractive index from and has substantially the same density as the insulating liquid. By electrowetting effects, it is possible to modify the curvature of the contact surface between the two liquids, as a function of a voltage V applied between the intermediate piece 42 and the cover 30, which form two electrodes. During this change in the curvature of the liquid-liquid interface, the edge of the interface between the conducting liquid 54 and the insulating liquid 52 moves along the conical surface 48. For example, the contact surface passes from the initial, e.g., concave shape, denoted by the reference A, to the convex shape illustrated by the dashed curve and denoted by the reference B. Thus, a light beam passing through the cell orthogonally to the plates 38 and 24 will be focused to a greater or lesser extent according to the applied voltage. In general, the conducting liquid comprises an aqueous liquid, and the insulating liquid comprises an oily liquid.
The “S”-shaped corrugated portions 23, 36 are able to deform when the liquids contained in the internal volume 15 expand, so as to limit the increase in internal pressure of the lens.
One possible limitation of such a lens is that a certain degree of deformation of the corrugated portions 23, 36 may result in a change in the shape of the lens housing, especially the distance separating the two transparent plates 24, 38. This may lead to the appearance of additional optical defects. Furthermore, the fact that the housing is deformable may make it difficult for the components of the housing to be precisely positioned, one with respect to another. Thus, it may prove difficult to keep the optical part of the lens centered with respect to a reference external to the lens. In addition, the production of the “S”-shaped portions 23, 36 requires specific stamping steps, which complicates the manufacture of such a lens. Thus, the present invention can be employed together with other measures for controlling the pressure inside the lens, such as the device described in the commonly owned earlier application, or it may be employed as the sole pressure controlling measure in a lens system.